Devices are utilized extensively in integrated circuits to protect the circuits from electrostatic discharge (ESD) events. Hereinafter these types of devices will be referred to as ESD protection structure. Besides the strength of the ESD protection structure, the most important parameter of the device is a triggering voltage (VT1) of the ESD protection structure. To describe the importance of this parameter in more detail refer now to the following discussion in conjunction with the accompanying Figures.
FIG. 1 is a first embodiment of a lateral depletion NMOS (LDNMOS) circuit 10 that is utilized as an ESD protection structure. In this embodiment the LDNMOS circuit 10 includes an LDNMOS device 12 in which the gate is coupled to ground and the source and drain are coupled to VDD and ground respectively. FIG. 2 is a second embodiment of an LDNMOS circuit 100 which is utilized as an ESD protection structure. In this embodiment, the LDNMOS circuit 100 includes a LDNMOS device 102 coupled to a plurality of diodes 106, 108 and 110 and also coupled to a resistor 104. As is seen the diode 106, 108 and 110 are coupled between VDD 114 and ground 116. Diodes 106 and 108 in one embodiment could be for example 18 volt diodes and diode 110 is a 6 volt diode. The gate of the LDNMOS device 102 is coupled between diodes 108 and 110. A resistor 104 is coupled between the gate of the LDNMOS device 102 and ground. In this embodiment the LDNMOS circuit 100 comprises an active clamp.
In a preferred embodiment the incoming ESD event is conducted to the LDNMOS device 102. To describe how the ESD protection structure operates to protect an integrated circuit, refer now to the following description in conjunction with the accompanying figures.
FIG. 3 is a top view of a circuit layout of a conventional design of an LDNMOS device 102′. The view includes a multi-finger device with body contact rows 202a-202e next to the source contact rows 204a-204e. This device 102′ includes alternating a plurality of source contact rows 204a-204h and body contact rows 202a-202e (SBS) and pluralities of drain contact rows 209a-209d (D) between gates. As is seen, the number of body contact rows 202a-202e is almost equal to the number of drain contact rows 209a-209d. The lower limit of the VT1 is determined by the supply voltage of the integrated circuit. The ESD protection structure does not conduct for voltages equal to or below the supply voltage. Therefore, VT1 must be larger than the supply voltage. The upper limit, on the other hand, is related to the breakdown voltage of the weakest device connected to the integrated circuit. VT1 must be lower than the breakdown voltage of this device 102. Otherwise, the device 102 is physically damaged during an ESD event.
Typically in a 0.35 μm smart power technology, the supply voltage is 25V and the breakdown voltage of the weakest device is 43V. Hence, for this type of technology, VT1 must be between 25V and 43V in order to guarantee a proper ESD protection. The device 102 shown in FIG. 3 does not operate effectively to provide ESD protection in this range. To describe the problem with a conventional ESD LDNMOS device, refer now to the following.
FIG. 4 is a cut away side view of the layout of the conventional LDNMOS device 102 of FIG. 3. FIG. 4A is a circuit schematic representation of the conventional LDNMOS device 102 of FIG. 4. To explain the operation of LDNMOS device 102 refer now to the following.
Referring to both FIGS. 4 and 4n, in the conventional design as is seen, there are a plurality of drains 206a-206h which receive the ESD event. This event can range from 0 volts to as high as 70 volts using this conventional technology. As is further seen, there are about as many source contacts 204a-204h as drain contact rows 209a-209d (FIG. 4).
In case of an ESD event the inherent parasitic bipolar device of the lateral DMOS transistor will bear the brunt of the current. This bipolar transistor—comprising the NDMOS' drain as collector, body as base and source as emitter—is triggered by impact ionisation. Carriers are generated inside the NDMOS' drain region when a large voltage with respect to the body is applied to the drain contact. The generated holes will travel to the next body contact row. In FIG. 4n, the body contact rows 202a-202d are depicted as conductors. The source-to-body diode will start to conduct a large current, if the voltage drop caused by this hole current below the source contact exceeds approximately 650 mV with respect to the source. This triggers the bipolar device and the drain voltage drops significantly. This phenomenon is referred to as snapback.
FIG. 5 shows the transmission line phasing (TLP) measurement for an active clamp with a multi-finger LDNMOS transistor using the conventional design of FIGS. 3 and 4. For the typical ESD device shown in FIGS. 3 and 4, the triggering voltage (VT1) exceeds the critical voltage of 43V for 0.35 μm smart technology. For example, the triggering voltage would be as high as 70 volts when utilizing the conventional LDNMOS device 102.
Accordingly, what is needed is an ESD protection structure in which the triggering voltage is reduced for certain applications. The ESD protection structure must be cost effective, simple to implement and adaptable to existing integrated circuits. The present invention addresses such a need.